Nikos Varelas University of Illinois at Chicago - SMU Physics...QCD in epInteractions s ≈300−320...

Nikos Varelas CTEQ Summer School 2005 1

Nikos Varelas

University of Illinois at Chicago

CTEQ Summer School

Puebla, Mexico

May 19 - 27, 2005


Introduction – ee, ep, pp processes– What is a jet – Jet algorithmsJet Characteristics– Jet energy profile– Differences between Quark and Gluon jets– Color coherence effectsJet Production at Tevatron– Challenges with jets– Inclusive jet cross sectionsOutlook


QCD in a Nutshell

Similar to QED BUT DifferentPointlike particles called quarksSix different “flavors” (u, d, c, s, t, b)Quarks carry “color” - analogous to electric chargeThere are three types of color (red, blue, green)Mediating boson is called gluon - analogous to photonColor charge is conserved in quark-quark-gluon vertexGluons carry two color “charges” and can interact to each other – very important difference from QED -from Abelian to non-Abelian theoryAt large distances: parton interactions become large (confinement)At small distances: parton interactions become small (asymptotic freedom)

u

d

u

Proton

gluons

quarks

Partons = quarks & gluons

Coupling constant → αs (analogous to α in QED)Free particles (hadrons) are colorless

QCD : Theory of StrongInteractions


The “Running” αsSU(3) gauge coupling constant (αS) varies with Q2, decreasing as Q2

increases:

Measurements of the strong coupling are made in many processes at different Q2, clearly establishing the running of αS.

Asymptotic freedom (αS 0 as Q2 ) Infrared slavery (αS as Q2 0)

→→ ∞→

→∞

Compilation of many experiments

Increase of αS as Q2 −> 0 means that color force becomes extremely strong when a quark or gluon tries to separate from the region of interaction (large distance ~ small Q2).

A quark cannot emerge freely, but is “clothed” with color-compensating quark-antiquark pairs.

Leading-Log Approximation

222

ln)233(12)(

Λ−=

QnQ

fs

πα

No free quarks or gluons only jets


QCD in e+e− Annihilations

Perturbative phaseαs<1 (Parton Level)

Non-perturbative phaseαs≥1 (Hadron Level)

LEP: 88 GeV < Ecm < 209 GeV

e+e− −> (Z0/γ)* −> hadrons

Z0/γe−

e+

q

q

HADRONIZATION

Hadrons

Z0/γe−

e+

q

q

Z0/γe−

e+

q

q

Z0/γe−

e+

q

qO(αs

0) O(αs1) O(αs

2)

Fixed Order QCD

SLC: Ecm = 91 GeV


Why do we Study Jets in e+e− ?

QCD StudiesMeasurements of αsFragmentation functionsColor/spin dynamicsQuark-gluon jet propertiesEvent shape variables (sphericity, thrust, …)

Searches for the HiggsSearches for new physics


e+e− Event Displays

e+e− −> µ+µ− e+e− −> qq e+e− −> qqg

Much cleaner events than hadron-hadron collisions


QCD in ep Interactions

HERAat GeV 320300 −≈s

squared massparton -electron )(ˆ

squared massproton -electron 2)(

nsferenergy tra fractional

fraction momentumparton 2

transfermomentum-4 )(e outgoingfor momentum-4 ),(e incomingfor momentum-4 ),(

2

22

2

222

sxkxPsxyQkPkPs

EEE

kPqPy

qPQx

kkqQEkEk

-

-

≈+=

=⋅≈+=

′−=

⋅⋅

=

⋅=

′−−=−=

′=′

=

kk

qe Z0/γ/W±

e,ν

k

k'

xPP

jet

proton remnantP

27.5 GeV

820-920 GeV


Why do we Study Jets in ep ?

Direct photoproduction

QCD StudiesMeasurements of αsFragmentation functionsParton Distribution FunctionsColor/spin dynamicsQuark-gluon jet propertiesEvent shapesInclusive- and Multi-jet productionRapidity Gaps/Diffraction

Searches for new physics

Resolved photoproduction


Proton-(Anti)proton CollisionsProton beams can be accelerated to very high energies (good)But the energy is shared among many constituents – quarks and gluons (bad)

Transversemomentum Tp≡

To select the interesting collisions: look for outgoing particles produced with high momentum perpendicular to the beamline(“transverse momentum”) → hard collisions• Hard collisions take place at small impact parameter and are more

accurately collisions between partons inside the two protons• Analog of Rutherford’s experiment• Forms the basis of the on-line event selection (“triggering”)


pp Interactions

(uud)

JetD

xa xbp pf σ

Jet

a b

c

d

f

DetectorpT = psinθ“Soft” collisions = small pT“Hard” collisions = large pT

θ

= =

(uud)

Tevatronat TeV 2=s

D

D(z,µF) is theFragmentationfunction

Proton Remnant

Parton Distribution Functionsfa/A(xa,µF): Probability function to find a parton of type a inside hadron A with momentum fraction xa

xa: fraction of hadron’s momentum carried by parton a

µF: related to the “hardness” of the interaction“Factorization Scale”

( )cdab →σ̂Partonic level cross section


EM E

ICD/MG E

FH E

CH E

“scattering angle”azimuth

pp Interactions cont’dφ

UnderlyingEvent

u

u

d

gq

q d

Hard Scatter

u

u

Complications from the e+e− due to:– Parton Distribution

Functions (PDFs)– “Colored” initial and final

states– Remnant jets - Underlying

event (UE)


EM E

ICD/MG E

FH E

CH E



UnderlyingEvent

u

u

d

gq

q d

Hard Scatter

u

u




event (UE)

��

��

��

��

��

��

��

��

��

��

��

�� !��

��"�� "��#�$

�%�� &�� '(��'�)��

DØ Event

ET1 ~ 620 GeVET2 ~ 560 GVMJJ ~ 1.2 TeV


Au+Au collision at RHIC Now this is a complicated event!


Why do we Study Jets in Hadron Colliders?QCD Studies

Measurements of αsFragmentation functionsParton Distribution FunctionsColor/spin dynamicsQuark-gluon jet propertiesEvent shapesInclusive- and Multi-jet productionRapidity Gaps/DiffractionProduction of Vector Bosons + jets

Study of heavy particles (e.g. top production)Searches for HiggsSearches for new physics

Quark sub-structure + …And much more …


Explanation of the blob’s

(uud)

JetD

xa xbp pf σ

Jet

a b

c

d

f

Detector

θ

= =

(uud)

D

(uud)

JetDD

xa xbp pf σσ

Jet

a b

c

d

f

Detector

θ

= =

(uud)

DD

pf =

(uud)

Parton Distribution Functions

xf(x,Qo) = Ao xA1 (1-x)A2 P(x)

small x behavior

large x behavior

in between

Parton Distribution Functions of the proton are measured at a some “hard scale” and evolved via pertrurbativeQCD to the “scale” of the interactionPDFs are determined doing Global Fits of data from DIS (Deep Inelastic Scattering), DY (Drell-Yan), Direct Photons, and production of jets


Explanation of the blob’s cont’d Particle Fragmentation Functions

Particle Fragmentation Functions DA/a (zA,µF) measure the probability of finding a particle of type A with momentum fraction zA of parent parton aFragmentation functions are determined doing Global Fits of data from DIS and e+e−

The “evolution” of the Fragmentation functions can be calculated by pQCD

(uud)

JetD

xa xbp pf σ

Jet

a b

c

d

f

Detector

θ

= =

(uud)

D

(uud)

JetDD

xa xbp pf σσ

Jet

a b

c

d

f

Detector

θ

= =

(uud)

DD


Explanation of the blob’s cont’d Hard Scattering Cross Section

(uud)

JetD

xa xbp pf σ

Jet

a b

c

d

f

Detector

θ

= =

(uud)

D

(uud)

JetDD

xa xbp pf σσ

Jet

a b

c

d

f

Detector

θ

= =

(uud)

DD( ) ( ) ( ) ⎟⎟

⎠

⎞⎜⎜⎝

⎛= ∑∫ 2

2

2

222

,

1

0

2 ,,,,ˆ,,RF

RsbaijFbjji

FaibaXQQppxfxfdxdxµµ

µασµµσ

What’s the deal with the various scales?µF is the factorization scale that enters in the evolution of the PDF’sand the Fragmentation functions (could be two different scales). It is an arbitrary parameter that can be thought as the scale which separates the long- and short-distance physicsµR is the renormalization scale that shows up in the strong coupling constantQ is the hard scale which characterizes the parton-parton interactionTypical choice: µF = µR = Q ~ pT/4 – 2pT of the jets

σX = (PDF’s for p and p) ⊗ (partoniclevel cross section)

Separate the long-distance pieces (PDF’s) from the short-distance cross section →Factorization


Detector

(uud)

JetD

xa xbp pf σ

Jet

a b

c

d

f

Detector

θ

= =

(uud)

D

(uud)

JetDD

xa xbp pf σσ

Jet

a b

c

d

f

Detector

θ

= =

(uud)

DD

Detector

Explanation of the blob’s cont’d

Typical Detector

CDF

Fermilab Accelerators

DØ


Tevatron RunsRun I

1992-1996ECM = 1.8 TeV~120 pb-1

(0.63 TeV ~600 nb-1)

Run IIa2002-2005ECM = 1.96 TeV~ 1.5 fb-1

Run IIb2006-2009ECM = 1.96 TeV~4-8 fb-1

Main Injector& Recycler

Tevatron

Chicago↓

⎯p source

Booster

⎯p

p

p ⎯p1.96 TeV

CDFDØ


Kinematics in Hadronic Collisions

Rapidity (y) and Pseudo-rapidity (η)θ

Particle

ppz

xy

θβθβ

cos1cos1ln

21ln

21

−+

=−+

≡z

z

pEpEy

Epy == βθβ wheretanhcos

2tanln

cos1cos1ln

21

then)(or 1limit In the

0θ

θθη

β

−=−+

=≡

<<→

=m

T

y

pm

LAB System ≠ parton-partonCM system

⇒

LAB

θ1

θ2

CM

θ∗


Kinematics in Hadronic Collisions cont’d

yEpyEEyEp

Tz

T

z

sinh cosh tanh

===Transverse Energy/Momentum

22222222zTyxT pEmpmppE −=+=++≡

Invariant Mass θsinppT ≡

)cos(cosh2)(2

))((

210,

212122

21

21212

12

21φη ∆−∆⎯⎯⎯ →⎯

⋅−++=

++≡

→

µµµµ

TTmm EEEEmm

ppppM

pp

2

1x1P x2P

Partonic Momentum Fractions)0(2 2,12,1 ==≡ ηxsEx TT

( )( ) sEeex

sEeex

T

T

21

21

2

1

ηη

ηη

−− +=

+=

1

1,0

212

21

<<

<<

xxx

xx

Tsxxs ba=→ ˆ(energy) CMParton 2


What are Jets ?


What are Jets ?

Whatever objects my jetalgorithm finds!


What are Jets ?

Colored partons from the hard scatter evolve via soft quark and gluon radiation and hadronization process to form a “spray” of roughly collinear colorless hadrons −> JETSThe hadrons in a jet have small transverse momenta relative to their parent parton’s direction and the sum of their longitudinal momentaroughly gives the parent parton momentum

Keep in mind that there are particles in a jet originating from other partonsin the event

Jets manifest themselves as localized clusters of energy

Jet

outgoing parton

Fragmentation process

Hard scatter

colorless states - hadrons -

R = +( ) ( )∆η ∆φ2 2coneR


What are Jets ?

Colored partons from the hard scatter evolve via soft quark and gluon radiation and hadronization process to form a “spray” of roughly collinear colorless hadrons −> JETSThe hadrons in a jet have small transverse momenta relative to their parent parton’s direction and the sum of their longitudinal momentaroughly gives the parent parton momentum

Keep in mind that there are particles in a jet originating from other partonsin the event

Jets manifest themselves as localized clusters of energy

Jet

outgoing parton


Hard scatter

colorless states - hadrons -

R = +( ) ( )∆η ∆φ2 2coneR

Jets

are th

e expe

riment

al sig

nature

s of q

uarks

and gl

uons


pp Interactions – Creation of Jets

p=

(uud)

(uud)

p=

{π,K,p,n,…}

Jet

BeamRemnants

BeamRemnants

Initial State Radiation(ISR)

Hadronization

Final State Radiation(FSR)

Detector


First Evidence for JetsFirst experimental evidence of quark-initiated jets in e+e- annihilations, SLAC-SPEAR at Ecm ~ 7 GeVG. Hanson et al. (MARK-I Collab), PRL 35, 1609 (1975)

Gluon-initiated jets were discovered in e+e- annihilations DESY-PETRA at Ecm > 15 GeVMARK-J Collab., PRL 43, 830 (1979); TASSO Collab., Phys. Lett. B86, 243 (1979); PLUTO Collab., Phys. Lett. B86, 418 (1979);JADE Collab., Phys. Lett. B91, 142 (1980)

e+e− → q q g


Jet AlgorithmsThe goal is to apply the “same” jet clustering algorithm to data and theoretical calculations without ambiguities


Jet AlgorithmsThe goal is to apply the “same” jet clustering algorithm to data and theoretical calculations without ambiguities

Jets at the “Parton Level” (i.e., before hadronization)

Fixed order QCD or (Next-to-) leading logarithmic summations to all orders

2 → 2 processLeading Order QCD

2-jet final state1 parton/jet

outgoing parton

Parton showering

Hard scatter

multi-jet final state


Jet AlgorithmsThe idea is to come up with a jet algorithm which minimizes the non-perturbativehadronization effects

Hard scatter


Jet

hadrons

outgoing parton

Parton showering+ Hadronization


Jet Algorithms

Jets at the “Detector Level”Calorimeter - clusters of energy “towers”Tracking - clusters of tracksCombination of detectors

Calorimeter

Calorimeter jet energy resolution:

%5%80⊕≈

TT EEσ


Jet Algorithms - Requirements

Experimental:– Detector independence - Can everybody implement this?– Minimization of resolution smearing/angle bias– Stability with Luminosity– Computational efficiency– Maximal reconstruction efficiency

Theoretical:– Infrared safety

• insensitive to “soft” radiation– Collinear safety

– Low sensitivity to hadronization– Invariance under boosts– Same jets at parton/particle/detector levels– Straight forward implementation

The full report of the Run II Jet algorithm specification is available at hep-ex/0005012


Jet Finders(Generic Recombination)

Define a resolution parameter ycut

For every pair of particles (i,j) compute the “separation”yij as defined for the algorithm

If min(yij) < ycut then combine the particles (i,j) into k– E scheme: pk=pi+pj massive jets– E0 scheme: massless jets

Iterate until all particle pairs satisfy yij>ycut

No problems with jet overlapLess sensitive to hadronization effects

2

2

vis

ijij E

My =

ji

jikk

jik

E

EEE

pppp

p+

+=

+=

i j


The JADE Algorithm )cos1(22

ijjiij EEM θ−=

cutvis

ijij y

EM

y <= )min()min( 2

2

Recombination: pk = pi+pj

Problems with this algorithm– It doesn’t allow resummation when ycut is small– Tendency to reconstruct “spurious” jets

i.e. consider the following configuration where two soft gluons are emitted close to the quark and antiquarkThe gluon-gluon invariant mass can be smaller than that of any gluon-quark and therefore the event will be characterized as a 3-jet one instead of a 2-jet event

(Evis is the sum of all particle energies)

x 3-Jet event √ 2-Jet event

ijθij


The Durham or “kT” Algorithm )cos1)(,min(2 222

ijjiij EEM θ−=

cutvis

ijij y

EM

y <= 2

2

)min(

),min(2

),min(2)2

1(1),min(2

:get we smallFor

222

222

222TjTi

ijji

ijjiij

ij

kkEEEEM ≈⎟⎟⎠

⎞⎜⎜⎝

⎛≈⎟

⎟⎠

⎞⎜⎜⎝

⎛+−−≈

θθ

θ

L

Recombination: pk = pi+pj

It allows the resummation of leading and next-to-leading logarithmic terms to all orders for the regions of low ycut

kT

√ 2-Jet event


The “kT” Jet Algorithm for Hadron Colliders

The kT jet algorithm successively merges pairs of partons, particles, calorimeter towers, or tracks in order of increasing relative transverse momentum (kT)It contains a parameter D (~0.5-1) that controls the termination of the merging and characterizes the approximate size of the resulting jetskT jets are infrared and collinear safeThere are no overlapped jetsEvery parton, particle, or detector tower is unambiguously assigned to a single jetNo biases from seed towersLess sensitive to hadronization effects


The “kT” Algorithm cont’d Input: List of Energy preclusters )2.0( ≈∆ preclusterR

2

22

,2

, ),min(DR

ppd ijjTiTij

∆=

2,iTii pd =

dij?

Move i to list of jets

Anyleft?

No

No

MinYes

Yes

Combine i+j

( ) ( )222jijiij yyR φφ −+−=∆

p p

Cone jetkT jet

jiij

jiij

EEE

ppp

+=

+=rrr

D ∼ 0.5-1

Output: List of jets )( DR ≥∆

p p


The Cone Jet Algorithm for Hadron CollidersA more intuitive representation of a jet that is given by recombination jet findersIt requires “seeds” with a minimum energy of ≤ 1 GeV (to save computing time)– Preclusters are formed by combining seed towers with their

neighbors within a cone of radius R is η−φ space– For each precluster the ET-weighted centroid is found and a new

cone of radius R is drawn around it– Iterate until stable solution is found

Jet cones may overlap so need to split/merge overlapping jets

Jet Seeds

Calorimeter ET

Merge if shared ET > 50-75% of min(ET1,ET2)

DØ - CDF


The DØ/CDF Jet Cone Algorithm for Run I In Run I: DØ and CDF used Snowmass (1990) clustering and defined angles via momentum vectors

DØ and CDF’s Angles:CDF’s ET:

DØ’s ET: ∑⊂

=Ji

iT

JT EE


The Cone Algorithm at 3-Parton Final States Apply Snowmass jet algorithm– Each parton must be within Rcone of centroid

The two partons must be within RsepxRcone of one another, where Rsep varies from 1 - 2 (Rsep=1.3 for DØ/CDF)– introduce ad-hoc parameter Rsep to control parton recombination in

the theoretical jet algorithm and simulate the role of seeds andmerging in the experimental algorithm

– it doesn’t generalize to higher orders Rcone

Rsep

Num

ber o

f jet

s

Rsep=1.3

DØ Cone Radius = 0.7

Delta_R

Run I

If jets from separate events are overlaid thenthey can be distinguished at 1.3xRcone=0.9 for 0.7 cone jets:


The Next Generation of Cone Algorithm Issues with the Snowmass Cone Algorithm

Sensitivity to infrared and collinear radiationNot proper 4-vector kinematics used in particle clustering and in calculating the final jet parameters

The Solution: Develop a “seedless” jet algorithm with proper kinematics

Infrared and Collinear safeVery computationally intensive

What was done: Develop the Midpoint jet algorithmApproximates the seedless algorithmInfrared safe Proper 4-vector kinematics used in all steps massive jets


The Midpoint Jet Cone Algorithm for Run II

Proto-jets are formed by combining seed particles with their neighbors within a cone of radius Rcone using the E-scheme

Particles = calorimeter towers, MC hadrons or partonsMidpoint seeds are added between proto-jets

Only midpoints between proto-jets satisfying the following conditions are considered: ∆R > Rcone and ∆R < 2×Rcone

Proto-jets found around seeds and midpoints can share particles

Merging/splitting procedure has to be appliedMerge jets, if more than a fraction f (50% for DØ, 75% for CDF) of min(pT1,pT2) of overlapping jets is contained in the overlap regionOtherwise split jets; assign the particles in the overlap region to the nearest jet

Keep only final jets with pT > threshold


Not a Perfect World After All!

Significant amounts of energy is not clustered in Midpoint algorithm

To reduce this effect CDF is using two values for the cone radius: one during search for stable cones (Rcone/2) and the second (Rcone) during the calculation of the final jet properties Search Cone Algorithm… this leads to other “features”

Overall this effect is quite small


Not a Perfect World After All!

Significant amounts of energy is not clustered in Midpoint algorithm

To reduce this effect CDF is using two values for the cone radius: one during search for stable cones (Rcone/2) and the second (Rcone) during the calculation of the final jet properties Search Cone Algorithm… this leads to other “features”

This effect is currently under study

between the two experiments

Overall this effect is quite small


Looking Inside the Jets

( )rΨ

The investigation of jet profiles gives insights into the transition between the parton produced in the hard process and the observed spray of hadronsJet profiles are sensitive to the quark/gluon jet mixture

Could separate quark and gluon jets in a statistical wayEnergy Flow (Jet Shape):

Measure the average transverse energy flow in sub-cones as a function of radial distance from the jet axis Use calorimeter towers or charged tracks

22 )()( φη ∆+∆=r


Jet energy profiles at Tevatron Run II

Gluon enriched jets (low-x/low-pT jets at Tevatron) are “broader” (i.e. less collimated, higher multiplicity of soft energy particles) than Quark-enriched jets (high-x/high-pT jets)


Jet energy profiles at Tevatron

Gluon enriched jets (low-x/low-pT jets at Tevatron) are “broader” (i.e. less collimated, higher multiplicity of soft energy particles) than Quark-enriched jets (high-x/high-pT jets)

Run II

gg

g

~ CF = 4/3

~ CA= 3

q qg 2

2

Quark & Gluon jets radiate proportional to their color factor:

><><

≡><

><≡

tymultiplicijet quark tymultiplicijet gluon

q

g

nn

r

At Leading Order (Ejet →∞):

With higher order corrections at Ejet (LEP) ~ 40 GeV:

25.249~ ==

F

A

CCr

5.1~r


Property of gauge theories. Similar effect in QED, the “Chudakoveffect” observed in cosmic ray physics in 1955

In QCD color coherence effects are due to the interference of soft gluon radiation emitted along color connected partonsTwo types of Coherence:– Intrajet Coherence

• Angular Ordering of the sequential parton branches in a partoniccascade

– Interjet Coherence• String or Drag effect in multijet hadronic events

Coherence

eeθγθ e

γe−

e+

γθθ eee >


Shower Development“Traditional Approach”

Shower develops according to pQCD into spray of partons until a scale of Q0 ~ 1 GeV.

Thereafter, non-perturbative processes take over and produce the final state hadrons

Coherence effects are included probabilistically (e.g., Angular Ordering, color dipole) and in the hadronization model

DGLAP Splitting Kernel


Shower Development“Traditional Approach”

Shower develops according to pQCD into spray of partons until a scale of Q0 ~ 1 GeV.

Thereafter, non-perturbative processes take over and produce the final state hadrons

Coherence effects are included probabilistically (e.g., Angular Ordering, color dipole) and in the hadronization model

DGLAP Splitting Kernel

“Local Parton Hadron Duality (LPHD) Approach”

Parton cascade is evolved further down to a scale of about Q0 ~ 250 MeV.

No hadronization process; Hadron spectra = Parton spectra

Simplicity. Only two essential parameters (ΛQCD and Q0) and an overall normalization factor


What is an Event Generator ?A “C” (of “Fortran”) program that generates events, trying to simulate Nature!Events vary from one to the next (random numbers)Expect to reproduce average behavior and fluctuations of real dataEvent Generators include:– Parton Distribution functions– Initial state radiation– Hard interaction– Final state radiation– Beam jet structure– Multiple Parton Interactions– Hadronization and decays

Some programs in the market:JETSET, PYTHIA, LEPTO, ARIADNE, HERWIG, COJETS...

Some parton-level only:VECBOS, NJETS, JETRAD, HERACLES, COMPOS, ALPGEN, PAPAGENO, MADGRAPH, EUROJET...

q

calo

rim

eter

jet

Time

q g

π K

part

onje

tpa

rtic

le je

t

hadrons

γ

CH

FH

EM

p

• •

p


Hadronization ModelsIndependent fragmentation– it is being used in ISAJET and COJETS– simplest scheme - each parton fragments independently following

the approach of Field and FeynmanString fragmentation– it is being used in JETSET, PYTHIA, LEPTO, ARIADNE

Cluster fragmentation– it is being used in HERWIG

String Fragmentation: Separating partons connected by color string which has uniform energy per unit length, corresponding to a linear quark confining potential

Cluster Fragmentation: Pairs of color connected neighboringpartons combine into color singlets.

Z0/γe−

e+

q

q

Z0/γe−

e+

q

q


e+e− interactions:Coherence Observations

First observations of final state color coherence effects in theearly ’80’s (JADE, TPC/2g, TASSO, MARK II Collaborations) (“string” or“drag” effect)

q

q

γ q

q

g

e+e− → q q ge+e− → q q γ

Depletion of particle flow in region between q and qjets for qqg events relative to that of qqγ jets.


gqqeeqqee →→ −+−+ vsγ

Particle flow in event planeφ (deg.)

1/N

dn/

dφ

Particle flow in event plane

Particle FlowParticle Flow X

1/N

dn/

dX

10-2

10-1

1

0 50 100 150 200 250 300 350

10-1

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Figure �� a� Charged particle �ow in the event plane for two�jet radiative events� and

three�jet multihadronic events� Error bars for the q�qg sample are smaller than the dots�

�b� Charged particle �ow with respect to the reduced angle X�

�

Gluon jet

String effect

gqqγqq

Data agree with analytic LPHD calculations

gluon jet or photon

φ

Leading quark jet

2nd quark jet

Leading quark jet 2nd quark jet


Coherence Observations cont’dpp interactions:

Colored constituents in initial and final state (more complicated that e+e−)

Emission from each parton is confined to a cone stretching to its color partner


Color Coherence – Wgqq →

Compare pattern of soft particle flow around jet to that around (colorless) W

Color connections


p=

(uud)

(uud)

p=

Color Coherence –

W

Event Plane

Transverse Plane

Wgqq →Soft gluon radiation preferentially emittedin the event plane

Initial-Final StateConstructive interference


p=

(uud)

(uud)

p=

Color Coherence –

W

Event Plane

Transverse Plane

Wgqq →Soft gluon radiation preferentially emittedin the event plane

Compare pattern of soft particle flow around jet to that around (colorless) W

Initial-Final StateConstructive interference

Color connections

1

1.5

2

(a)

Data

PYTHIA (AO ON, SF)

(b)

Data

PYTHIA (AO OFF, SF)

1

1.5

2

0 π/4 π/2 3π/4

(c)

Data

PYTHIA (AO OFF, IF)

Jet/W

Tow

er M

ultip

licity

Rat

io

0 π/4 π/2 3π/4 π

(d)

Data

MLLA+LPHD

Far BeamNear Beam

Run I

Ratio of partile multipliity around the Jet to that around the W


Jet Production @ TevatronMotivation:

– Search for breakdown of the Standard Model at shortest distances• At Tevatron energies:

– Search for new particles decaying into jet final states– Search for quarks substructure– Constrain gluon density at high x– Precision studies of QCD

mp

GeVp

T

T

19104~GeV 500

fmMeV 200~c~distance

500~

−×⋅

⇒h

World’s best

microscope !


Challenges with JetsTriggering on Jets– reduce rate from ~106 to ~ tens of Hz

• multiple triggering stages; Level-1,2,3– implement fast/crude jet clustering algorithms for L1/2

Selection of a Jet Algorithm– at detector, particle, parton/NLO++ level

Jet Reconstruction, Selection, and Trigger EfficienciesJet Calibration– underlying event definition (subtract or not?)– out-of-cone showering effects– correction back to particle jet or original parton?

Jet Resolution– difficulties with low-ET region and near reconstruction threshold

Simulation of Jet/Event/Detector Characteristics– precision of detector modeling vs CPU time– ability to overlay zero/minimum-bias events from data – tuning of fragmentation model, selection of PDF, hard scale

parameter Q, …– Interface a ME event generator with a parton-shower simulation


High-ET Jet Production

1P1P

2P2P

11Px 11Px

22 Px 22 Px

1jet 1jet

2jet 2jet

( )sασ̂ ( )sασ̂

( )1/ xf Aa ( )1/ xf Aa

( )2/ xf Bb ( )2/ xf Bb

Quark substructure ?PDFs ?

ET

d2σdET dη

Central η region

Significant gluon contribution at high pT

ddP

dx f x dx f xddPT

aa b

a A a b b B bT

σµ µ

σ≈ ∫∑ ∫

,/ /( , ) ( , )

$

ddP

ab cd MT

s

N

N

N

$( )

( )σ α µπ

→ ≈⎛⎝⎜

⎞⎠⎟∑

2

Currently 3-jet production @ NLO


Resent results from Tevatron

(GeV)Tp50 100 200 400

dy (

pb/G

eV)

T/d

pσ2 d

-410

-310

-210

-110

1

10

210

310

410

510

|y| < 0.4 (x10)

0.4 < |y| < 0.8

NLO QCD

CTEQ6.1M

T = pFµ = Rµ

= 1.96 TeVs-1

L = 378 pb

DØ Run II preliminary

Run IIMid-point jet algorithm kT jet algorithm

DØ

NLOJET++


Resent results from Tevatron cont’dRun II kT jet algorithmMid-point jet algorithm

0.8

0.9

1

1.1

1.2

1.3

0.0 < |y| < 0.4

rela

tive

chan

ge o

f inc

l. je

t cro

ss s

ectio

n

0.8

0.9

1

1.1

1.2

1.3

100 200 300 400 500 600

0.4 < |y| < 0.8

pT (GeV)

non-perturbative corrections

PYTHIA 6.225underlying eventhadronization

Hadronization + Underlying event/multiple-parton scatteringcorrections using Pythia applied to NLO QCD


⎟⎟⎠

⎞⎜⎜⎝

⎛

=

→←

TeV 14s

ppThe far future...

distance resolution ~10-20 m

16 orders of magnitude drop


Jets celebrate their 30th year since first observed in e+e-

Jets have provided the means to study the SM and explore possibilities beyondThere are still issues to be resolved with the jet algorithms

See the current effort in the TeV4LHC WorkshopMore sophisticated jet algorithms are under development

Tevatron Run II is setting the stage for jet physics at LHC

CMSCMS

http://conferences.fnal.gov/tev4lhc/

Nikos Varelas University of Illinois at Chicago - SMU Physics...QCD in epInteractions s ≈300−320...

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Transcript of Nikos Varelas University of Illinois at Chicago - SMU Physics...QCD in epInteractions s ≈300−320...